The present invention generally relates to optoelectronic devices and more particularly relates to low resistance optoelectronic devices.
Vertical cavity surface emitting lasers (VCSELs) have been widely and rapidly adopted into Gigabit Ethernet and other applications. VCSELs are particularly suitable for multi-mode optical fiber local area networks due to their reliability, reduced threshold current, circular output beam, and inexpensive and high volume manufacture. In addition single mode lasers have advantages in terms of high speed data transmission due to the well defined rise and fall time of the single optical mode.
A principal characteristic of a VCSEL is that it emits beams vertically, i.e. in a direction normal to the p-n junction of the semiconductor wafer from which it was fabricated. Historically, VCSELs have been fabricated using crystalline growth techniques to deposit many layers of semiconductor material upon a substrate. These lasers include highly reflective surfaces above and below an active layer, forming a laser cavity perpendicular to the active layer plane. In III-V semiconductor light emitting devices, the active region is typically disposed between n-type and p-type semiconductor regions. Upon application of an electrical potential, holes enter the active region from the p-type semiconductor material and recombine with electrons that enter the active region from the n-type semiconductor material, and photons are emitted.
Conventional VCSEL designs utilize a thin active region, typically on the order of one wavelength of the emitted light, to achieve a low threshold current as well as longitudinal (or axial) mode control. However, thin active regions typically have a single pass optical gain of approximately 1%, so that upper and lower mirrors having reflectivities greater than about 99% are required to achieve lasing. Conventional VCSEL designs often utilize semiconductor distributed Bragg reflectors (DBRs) to achieve the required reflectivity. DBRs provide the necessary reflectivity but have the disadvantage of being highly resistive and in operation may cause significant levels of self heating.
Due to its relatively small current carrying volume, heating in VCSELs is an important issue. For example, the operating performance of a VCSEL (slope efficiency and threshold) typically varies as a function of temperature. In addition, long term laser reliability may also be compromised in high resistivity devices.
VCSEL heating may be further exaggerated in designs that incorporate current constriction techniques such as, for example, mesas, oxide apertures or ion implantations in the upper mirror for single transverse mode operation. Current constrictions confine the current flowing in the upper mirror of these designs so that the current density in the constricted region is orders of magnitude higher than in the bottom or un-constricted mirror. Thus, the majority of the voltage drop and heat generation occurs in the top or constricted mirror.
Conventional VCSEL structures frequently have a p-type upper DBR that uses holes as the majority current carrying species. Therefore, previous attempts to lower the electrical resistance of mirrors have primarily focused on the p-type DBR. However, in the AlGaAs system, the minimum in the valence energy band for p-type material varies in a smooth almost linear fashion in accordance with the aluminum composition within the material. It is therefore a relatively straightforward matter to design low resistance p-type DBRs.
However, p-type material is more difficult to work with than n-type material, especially for VCSELs that emit at long wavelengths, such as in the 1.2 to 1.6 xcexcm regime. The p-type material tends to be operationally inferior to corresponding n-type material with regard to carrier mobility, overall electrical efficiency, and free carrier optical absorption at these wavelengths. Therefore, long wavelength VCSEL designs typically reduce or eliminate the use of p-type mirror layers to minimize optical loss through the p-type material.
In one aspect of the present an optoelectronic device includes an active region sandwiched between an upper mirror and a lower mirror, wherein at least one of the upper and lower mirrors is formed from alternating layers of high index and low index of refraction semiconductor material with a step graded interfacial transition layer there between.
In a further aspect of the present invention an optoelectronic device includes an active region sandwiched between a first mirror and a second mirror, wherein the second mirror comprises a plurality of mirror periods formed from alternating layers of a first material having a first index of refraction and a second material having a second refraction with an interfacial transition layer between the first and second materials and a tunnel junction formed in said second mirror for injecting holes into said active region.
In another aspect of the present invention an optical subassembly includes an electrical package containing a VCSEL having at least one mirror comprised of a plurality of mirror periods wherein at least a portion of the mirror periods are formed from alternating layers of a first material having a first index of refraction and a second material having a second index of refraction. The electrical package may further include a photodetector for monitoring power of the VCSEL. In addition, a housing may be attached to the electrical package, the housing including a ferule for aligning a fiber with an optical path carrying light from the VCSEL.